As has been pointed out elsewhere, Obama was probably talking about his great-uncle Ralph Dunham, who seems to have been inducted into the Army, although nothing else is coming up online in regards to his service other than a stray reference to his service number being shared with another Kansas inductee.

Of course, the Kansas induction records also show that Obama's grandfather was inducted six months after Pearl Harbor, not the next day, but that's the sort of nostalgic gloss which tends to get applied to family WWII stories, the same sort that turned my own great-uncle Lucky into a "paratrooper" in family lore, which doesn't really agree with the battalion number listed on his gravestone...